Development of an aerated struvite crystallization reactor for phosphorus removal and recovery from swine manure

Abstract:

Declining phosphate reserves and stricter regulations regarding wastewater discharge have increased the need for phosphorus removal and recovery. Crystallization is a promising option since P would not only be removed from the waste stream, but could also be recovered as struvite, a potential fertilizer. The purpose of this research was to achieve effective P removal and recovery through struvite precipitation from agricultural wastewater with minimal chemical input. It was found that raw swine manure had the potential for P removal and recovery through struvite precipitation by raising the pH through aeration without any Mg2+ amendment. This led to the development of a side-stream continuous 12 L reactor design with a novel combination of fluidized seedbed and aeration for pH increase. Synthetic feed was used to optimize the operational parameters of the reactor system. It was found that for a 100 ml/min influent rate, an aeration and recycle rate combination of 4.5 – 7 LPM and 700 ml/min was sufficient for increasing and maintaining the reactor pH from 6.7 to between 7.6 and 8.0. Significant P removal was achieved in six h runs without a seedbed (91 – 92%), while neither the struvite nor sand seedbeds improved P removal (91 – 96%). Struvite was recovered in all runs, with additional Ca2+ precipitation in the seedbed runs. Long-term runs showed that operation of the reactor was possible for an extended period of time, up to 46 h without any major adjustment. The average P removal was 85 – 88%, and precipitate collected after 24 h was found to be mainly struvite, while the final precipitate also contained calcite. This study has demonstrated the technical feasibility of an aerated crystallization reactor system for struvite removal and recovery from synthetic swine wastewater. By avoiding chemical amendments this reactor system has eliminated a significant portion of the operational costs found in comparable systems. The major obstacle for achieving system stability and consistency was scaling.